Most fluids have a resistance to flow which is characterized as viscosity. The higher the resistance to flow, the higher the viscosity. For example, flow is measured in most materials by applying a shear force or stress and measuring the resulting shear rate. Viscosity can thus be characterized as the ratio of the shear stress and the shear rate. The ratio of applied shear rate resulting from the applied shear stress is known as the absolute viscosity, or dynamic viscosity. Viscometers that measure absolute viscosity apply a shear force and measure the resulting drag or damping. Other viscometers measure viscosity by timing the flow of a given volume of fluid without imposing external forces. The flow is the result of the hydrostatic head of the material. This type of viscosity measurement is kinematic viscosity. The relationship between absolute viscosity (.eta.) and kinematic viscosity (.nu.) is given by: EQU .nu.=.eta./.rho.
where .rho. is the density of the fluid.
However, viscosity has been generally difficult to measure for several reasons relating to the physical nature of the fluid under investigation such as the shear rate or viscosity gradient, temperature, and density. Shear rate due to applied forces such as the forces applied by mixers, pumps, gravity, and the like effects fluid viscosity differently for different fluids. The viscosity of fluids such as water or oil remains constant regardless of changes in shear rate and are more predictable in handling. Other fluid viscosities change with a change in shear rate. The fluid characteristics are not proportional to the change in shear rate (i.e., paint becomes thinner when stirred).
In addition, temperature greatly effects viscosity by altering the kinetic energy of a fluid. Typically, as fluids become warmer, viscosity decreases. For this reason, a viscosity index of the fluid is used to characterize the rate of change in viscosity. Viscosity can change by more than +/-5% with a 1 degree C. change in temperature. Likewise, density effects the viscosity when the effects of gravity are taken into account (i.e., the weight of the fluid above an orifice effects the speed with which the fluid flows through the orifice).
Various viscosity measurement technologies are known in the art. One type of known mechanical viscometer utilizes efflux cups to measure kinematic viscosity. An operator fills the cup of the instrument, then times the period for the cup to empty. Bubble time, falling needle, falling ball, and falling element-type instruments are other examples of low-cost time-based measurement. In these instruments, the operator fills a chamber with the fluid sample, then times the period for a bubble to rise or a needle to fall. While these instruments are useful for simple qualitative testing, the timing of the test is susceptible to a degree of variability. Non-Newtonian and particulate-laden fluids are also difficult to measure with these instruments. Moreover, a rather large quantity of fluid is required to satisfactorily perform the test.
Higher accuracy devices are available primarily in laboratory setups such as those which use glass capillary action. While they are generally more elaborate than the lower cost mechanical systems, they nevertheless operate on a time-based measurement. In addition, complex setup of the instrumentation is usually required.
Other viscometers utilize electromechanical sensors to measure absolute viscosity. These instruments sometimes use rotating cups, piston or vibratory arms, or rotating spindles to apply a known shear force to the fluid and monitor the response of the system. Such instruments still have difficulty resolving lower viscosity ranges, and require insertion into the liquid being measured or require relatively large fluid samples. For example, one type of electromechanical sensor employs coils that generate a magnetic force to move a piston back and forth. This instrument analyzes the travel time of the piston to measure absolute viscosity. However, clean up can be difficult depending on the design of the instrument.
Still other oscillating types of viscosity measurement devices use spheres or probes that oscillate at a desired rate. Circuitry is disposed to measure the dampening of oscillation to measure absolute viscosity. Although a wide range of viscosity measurement is possible, portability of these devices is often difficult. Likewise, these instruments often require a large sample volume to perform the measurement.
While these systems perform satisfactorily in laboratory or for the particular capacities for which they are intended, they suffer from requiring relatively large fluid samples and are primarily embodied in in-situ laboratory setups and the like. Thus, it is desirable to have a portable viscometer that is capable of quick response with small fluid sample requirements, while being highly accurate.